I have no idea what happened here, but it may be that test are sometimes loaded a few days later.

For example here in Argentina he had a huge peak on October 1st 2020 https://ourworldindata.org/explorers/coronavirus-data-explor... but it's not a real peak. Some death reports were misplaced and found later and loaded into the system that day (or the previous day, I'm not sure about the details).

Edit: Apparently the timestamp is the date of download. More details in https://news.ycombinator.com/item?id=29892505